Ran into a small issue with the patch file (it rejected some of the changes to phpbb/session.php). I checked the file and it looks like it updated, but I'll upload a fresh one just to be on the safe side. Other than that, update when flawlessly

We have been hard at work to ensure 3.1 Ascraeus is also the most stable and secure release of phpBB. A professional security audit performed by SektionEins found phpBB 3.1 Ascraeus to be solid, due to the examplary execution of strict coding guidelines.

This vulnerability was not found during this check?
Bizarre After they fall over release.

The awesome thing about us using github for development is that you get to use all their cool tools. For example: https://github.com/phpbb/phpbb/compare/release-3.1.0...release-3.1.1 will show you the commits that occurred between the two releases. Click on Files changed at the top there and you can see the aggregate changes per file.

Alternatively, you can grab a copy of 3.1.0 and 3.1.1 and run the directories you care about through your favorite file differ.